1. Field of the Invention
The present invention relates to an optical recording medium and an optical recording and reproduction device, more particularly an optical recording medium used at a near field which is able to prevent heat buildup at the surface of the optical recording medium due to focusing of light when recording or reproducing and the consequent signal loss or damage to the optical recording medium and an optical recording and reproduction device including the same.
2. Description of the Related Art
Up to now, a hard disk or other magnetic recording medium has been used in a state with a head for recording and reproduction brought into extremely close proximity to the disk or other medium for the purpose of obtaining good signal characteristics. As opposed to this, a phase change type optical disk, magneto-optical disk, or other optical recording medium is used in a state with the optical system or head for recording and reproduction separated from the recording medium by a predetermined distance.
However, in recent years, in the devices used for optical recording media, the system of bringing the optical system or head into close proximity, for example, 200 nm with the disk (near field) has begun to be employed for the purpose of increasing a numerical aperture (NA) of the optical system and thereby increasing a recording density of the disk.
As an optical recording medium device used at the near field, for example, there are an optical hard disk structured with a lens mounted on a slider, an optical disk device with a lens made movable by an electromagnetic actuator, etc. In these devices, light for recording and reproduction is focused on the recording medium by an optical system comprised of a plurality lenses including at least an objective lens and a solid immersion lens (SIL). Due to this, an NA of over 1 has been obtained.
FIG. 1 is a schematic view of a hard disk. The disk 11 is structured with a recording layer 13 and a lubrication film 14 stacked on a substrate 12. A recording and reproducing head 15 for changing the magnetization of the recording layer 13 is mounted on a slider 16 and movable in the direction of the disk plane. The lubrication film 14 is provided for preventing abrasion of the head 15 and the disk 11. The lubrication film 14 is formed, for example, by coating a fluorine compound. In the case of an optical disk, consideration of optical conditions is required for a layer formed on a recording layer, but the lubrication film 14 of a hard disk does not require consideration of optical conditions. Therefore, it can be relatively easily formed.
FIG. 2 is a schematic view of an optical disk with a long distance between an optical system or head and the disk (far-field optical disk). The optical disk of FIG. 2 is for example a phase change type optical disk or a magneto-optical disk and is structured with a dielectric protective layer 22, a recording layer 23, a dielectric protective layer 24, a reflective film 25, and a resinous protective layer 26 sequentially stacked on a substrate 21.
In the case of a phase change type optical disk, a material changing in phase by focusing of light is used for the recording layer 23. Both surfaces of the recording layer 23 are protected by the dielectric protective layers 22, 24 comprised of for example ZnS—SiO2. These surfaces are further protected by the substrate 21 or the resinous protective layer 26.
In the case of a magneto-optical disk, a material changing in magnetization state by focusing of light is used for the recording layer 23.
The optical disk of FIG. 2 has a much longer distance between a lens 27 and the disk than that of a hard disk. A film for dealing with friction or collision between the lens 27 (or head) and the disk is usually preferable, but not necessary.
FIG. 3 is a cross-sectional view of an optical disk used in the near field. It is structured by a reflective layer-32, a second dielectric layer 33, a recording layer 34, and a first dielectric layer 35 sequentially stacked on a substrate 31. In the case of the optical disk shown in FIG. 2, light is focused from the side at which the transparent substrate 2I is formed. On the other hand, in the case of the optical disk for near-field use shown in FIG. 3, light is focused from the side at which the first dielectric layer 35 is formed. Due to this, the increase in the coma along with an increase in the NA is moderated.
In the optical disk of FIG. 3, the four layers of the first dielectric layer 35, the recording layer 34, the second dielectric layer 33, and the reflective layer 32 are optimized in design for obtaining good signal characteristics for light striking the disk surface perpendicularly.
On the other hand, in the case of a near-field optical disk device having a short distance between the head and the disk as described above, the risk of collision of the lens and the rest of the optical system (or head) with the disk becomes extremely high. However, it is very difficult to uniformly coated a lubricating substance such as used for the lubrication film 14 of the hard disk on the surface of an optical disk to form a thin film satisfying the optical conditions. Also, in the case of a near-field configuration, the fluorine material used for the lubrication film 14 of a hard disk cannot be used because the refractive index is too low. There are few other suitable materials.
When an AR coating is provided on the surface of the lens, once the AR (Antireflecting) coating at the lens side is damaged due to collision, the recording and reproduction influenced by the damage at all times. That is, a change of the optical characteristics of the entire device is caused. However, it is difficult to find a suitable coating material resistant to damage by collision as the material for the AR coating.
Further, according to the conventional near-field optical disk shown in FIG. 3, as the protective layers 33, 35 of the recording layer 34, a ZnS—SiO2 layer having a low heat conductivity is often formed. Therefore, when focusing laser light at the time of recording and/or reproduction, heat builds up between the optical system and the recording layer. The heat causes the problems of loss of signals recorded on the optical disk or damage to the disk itself.
FIG. 4A shows a result of calculating the temperature rise due to focusing of laser light when making the optical disk the film configuration shown in FIG. 3. In FIG. 4A, the temperature of the recording layer is plotted against time after focusing of the laser light. As shown in FIG. 4B, the disk surface was set as an x-y plane, the beam spot was set as the origin O, and the direction of the optical axis of the laser light was set as a z-axis. The direction x of beam travel is the direction of relative movement of the beam spot on the disk along with disk rotation. Therefore, y corresponds to a distance from the beam spot in the direction of the disk radius. The calculation was performed setting y as 0 nm, 300 nm, and 500 nm. The intensity of the laser light was assumed to be 0.2 mW or the same level as the reproducing light.
The specific film configuration was made as follows (below, this film configuration is used as a comparative example for the present invention). The reflective layer 32 was made an Al-alloy layer of a thickness of 120 nm. The second dielectric layer 33 was made a ZnS—SiO2 layer having a refractive index n=2 and a thickness of 20 nm. The recording layer 34 was made a Ge—Sb—Te layer having a refractive index n=3.9, a quenching coefficient k=3.5, and a thickness of 20 nm. The first dielectric layer 35 was made a ZnS—SiO2 layer having a refractive index n=2 and a thickness of 100 nm. An optical system (lens) arranged in proximity to the optical disk was made a refractive index n=1.8.
As shown in FIG. 4A, although the intensity of the laser light is sufficiently smaller than the intensity of the recording light, the recording layer reaches a high temperature. Also, during a certain time until the rising temperature goes down, heat is built up at the recording layer.
Generally, it is necessary to reduce the distance between the lens and the disk for increasing the reproduced signal level. When a recording and reproducing system, in which the lens and the disk are brought into extremely close proximity, is used, the recording layer is easily damaged due to collision of the lens with the surface of the disk.
However, according to the configuration shown in FIG. 3, the first dielectric layer 35, that is, a thin layer of for example ZnS—SiO2 or SiN, is formed on the outermost layer of the disk. Therefore, when the lens and the disk collide, the first dielectric layer 35 and the recording layer 34 below the layer 35 are readily damaged.
Also, when only the extremely thin first dielectric layer 35 is formed on the recording layer 34, local light absorption easily occurs at the surface of the disk. If ablation occurs due to laser light at the time of recording or reproduction, the disk will be damaged and the lens contaminated by the deposition of the disk material on the lens surface.